Prior to the present invention, it has been known that the processability of polypropylene improves by introducing into the polypropylene, after its manufacture, a small amount of a peroxide compound. The peroxide compound is typically added to polypropylene particulates or pellets prior to their introduction to an extruder, sometimes in an inert gas, to melt them by heat and/or the mechanical energy of the screw or mixer, and to extrude them as pellets, film, or sheet having controlled, predictable flow properties.
Peroxide compounds are referred to in U.S. Pat. No. 3,144,436 as free radical initiators; in this patent they are employed in extruders to modify the melt index of the product.
Aliphatic peroxides are employe, also with crystalline polypropylene powder, in U.S. Pat. No. 3,887,534, modifying the intrinsic viscosity and the melt flow.
Controlled oxidative degradation of polypropylene is also achieved through the use of certain other peroxides, in U.S. Pat. No. 3,940,379; this patent emphasizes the essentially color and odor-free characteristics of the product obtained through minimal thermal degradation together with maximum oxidative degradation.
Several patents teach the cross-linking of polyethylene through the use of peroxides--see U.S. Pat. Nos. 3,846,396, 3,725,455, 3,917,745, and 3,963,673.
One of the peroxide compounds which we employ, 3,6,6,9,9 pentamethyl-3 n-propyl-1,2,4,5 tetraoxacyclononane, is included in the generic formula of a group of peroxides disclosed in U.S. Pat. No. 4,271,279 as useful for cross-linking polyethylene. A compound also within that generic formula is shown in U.S. Pat. No. 4,451,589 to be useful for improving the processability of propylene copolymers.
The reader may also be interested in U.S. Pat. No. 4,460,750 which shows the formation of transparent film of ethylene copolymers by treatment of peroxides.